1. Field of the Invention
The present invention generally relates to magnetoelectronic devices, and more particularly to a spin valve transistor (SVT) having an insulating hard bias stabilization.
2. Description of the Related Art
A spin valve transistor is a vertical spin injection device which has spin oriented electrons injected over a barrier into a free layer, and is used as a magnetic field sensor device. Those spin oriented electrons that are not spin scattered continue and then traverse a second barrier. The current over the second barrier is referred to as the magneto-current. Conventional devices are constructed using silicon wafer bonding to define the barriers.
Conventional spin valve transistors are constructed using a traditional three-terminal framework having an emitter/base/collector structure of a bipolar transistor. SVTs further include a spin valve on a metallic base region, whereby the collector current is controlled by the magnetic state of the base using spin-dependent scattering.
Magnetoresistive (MR) sensors have also been proposed to be incorporated as the read sensor in hard disk drives as described in U.S. Pat. Nos. 5,390,061 and 5,729,410, the complete disclosures of which are herein incorporated by reference. A magnetoresistive sensor detects magnetic field signals through the resistance changes of a read element, fabricated of a magnetic material, as a function of the strength and direction of magnetic flux being sensed by the read element. The conventional MR sensor, such as that used as a MR read head for reading data in magnetic recording disk drives, operates on the basis of the anisotropic magnetoresistive (AMR) effect of the bulk magnetic material, which is typically permalloy (Ni81Fe19). A component of the read element resistance varies as the square of the cosine of the angle between the magnetization direction in the read element and the direction of sense current through the read element. Recorded data can be read from a magnetic medium, such as the disk in a disk drive, because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance of the read element and a corresponding change in the sensed current or voltage.
The use of an SVT device such as a MR read head has also been proposed, as described in U.S. Pat. No. 5,390,061. One of the problems with such a MR read head, however, lies in developing a structure that generates an output signal that is both stable and linear with the magnetic field strength from the recorded medium. If some means is not used to maintain the ferromagnetic sensing layer of the SVT device (i.e., the ferromagnetic layer whose moment is not fixed) in a single magnetic domain state, the domain walls of magnetic domains will shift positions within the ferromagnetic sensing layer, causing noise which reduces the signal-to-noise ratio and which may give rise to an irreproducible response of the head. A linear response of the head is required. The problem of maintaining a single magnetic domain state is especially difficult in the case of an SVT MR read head because, unlike an AMR sensor, the sense current passes perpendicularly through the ferromagnetic layers and the tunnel barrier layer, and thus any metallic materials in direct contact with the edges of the ferromagnetic layers will short circuit the electrical resistance of the read head.
FIG. 1(a) is a simplified block diagram of a conventional magnetic recording disk drive for use with the SVT MR read head, and FIG. 1(b) is a top view of the disk drive of FIG. 1(a) with the cover removed. Referring first to FIG. 1(a), there is illustrated in a sectional view a schematic of a conventional disk drive of the type using a MR sensor. The disk drive comprises a base 510 to which are secured a disk drive motor 512 and an actuator 514, and a cover 511. The base 510 and cover 511 provide a substantially sealed housing for the disk drive. Typically, there is a gasket 513 located between base 510 and cover 511 and a small breather port (not shown) for equalizing pressure between the interior of the disk drive and the outside environment. A magnetic recording disk 516 is connected to drive motor 512 by means of hub 518 to which it is attached for rotation by the drive motor 512. A thin lubricant film 550 is maintained on the surface of disk 516. A read/write head or transducer 525 is formed on the trailing end of a carrier, such as an air-bearing slider 520. Transducer 525 is a read/write head comprising an inductive write head portion and a MR read head portion. The slider 520 is connected to the actuator 514 by means of a rigid arm 522 and a suspension 524. The suspension 524 provides a biasing force which urges the slider 520 onto the surface of the recording disk 516. During operation of the disk drive, the drive motor 512 rotates the disk 516 at a constant speed, and the actuator 514, which is typically a linear or rotary voice coil motor (VCM), moves the slider 520 generally radially across the surface of the disk 516 so that the read/write head 525 may access different data tracks on disk 516.
FIG. 1(b) is a top view of the interior of the disk drive with the cover 511 removed, and illustrates in better detail the suspension 524 which provides a force to the slider 520 to urge it toward the disk 516. The suspension may be a conventional type of suspension, such as the well-known Watrous suspension, as described in U.S. Pat. No. 4,167,765, the complete disclosure of which is herein incorporated by reference. This type of suspension also provides a gimbaled attachment of the slider which allows the slider to pitch and roll as it rides on the air bearing surface. The data detected from disk 516 by the transducer 525 is processed into a data readback signal by signal amplification and processing circuitry in the integrated circuit chip 515 located on arm 522. The signals from transducer 525 travel via flex cable 517 to chip 515, which sends its output signals to the disk drive electronics (not shown) via cable 519.
A conventional SVT is described by Jansen, R. et al., Journal of Applied Physics, Vol. 89, No. 11, June 2001, “The spin-valve transistor: Fabrication, characterization, and physics,” the complete disclosure of which is herein incorporated by reference. FIG. 1(c) illustrates a conventional SVT having a semiconductor emitter region, a collector region, and a base region which contains a metallic spin valve. The semiconductors and magnetic materials used may include an n-type Si as an emitter and collector, and a Ni80Fe20/Au/Co spin valve in the base region. Energy barriers, also referred to as Schottky barriers are formed at the junctions between the metal base and the semiconductors. It is desirable to obtain a high quality energy barrier at these junctions having good rectifying behavior, therefore, thin layers of magnetic materials, such as Pt and Au, are used at the emitter and collector regions, respectively. Moreover, these thin layers separate the magnetic layers from the semiconductor materials.
A conventional SVT functions when current is introduced between the emitter region and the base region (denoted as IE in FIG. 1(c)). This occurs when electrons are injected over the energy barrier and into the base region, such that the electrons are perpendicular to the layers of the spin valve. Moreover, because the electrons are injected over the energy barrier, they enter the base region as non-equilibrium hot electrons, whereby the hot-electron energy is typically in the range of 0.5 and 1.0 eV depending upon the selection of the metal/semiconductor combination.
The energy and momentum distribution of the hot electrons change as the electrons move through the base region and are subjected to inelastic and elastic scattering. As such, electrons are prevented from entering the collector region if their energy is insufficient to overcome the energy barrier at the collector side. Moreover, the hot-electron momentum must match with the available states in the collector semiconductor to allow for the electrons to enter the collector region.
The collector current IC, which indicates the fraction of electrons that is collected in the collector region is dependent upon the scattering in the base region, which is spin dependent when the base region contains magnetic materials. Furthermore, an external applied magnetic field controls the total scattering rate, which may, for example, change the relative magnetic alignment of the two ferromagnetic layers of the spin valve. The magnetocurrent (MC), which is the magnetic response of the SVT can be represented by the change in collector current normalized to the minimum value as provided by the following formula: MC=[IPC−IAPC]/IAPC, where P and AP indicate the parallel and antiparallel state of the spin valve, respectively.
The drawbacks of some of the conventional devices are that the magnetic state of the device during non-operation, or during the non-active state, is not known. This causes the free layer to “wander”, wherein the magnetization of the free layer is not oriented in a proper position resulting in an unstable device. Therefore, there is a need for a novel spin valve transistor which overcomes the limitations of the conventional devices.